1. Field of the Invention
The present invention relates to a machine designed for direct or indirect application of a liquid or viscous coating medium onto a moving surface.
2. Description of the Related Art
Coating machines for direct or indirect application of a liquid or viscous coating medium onto a moving surface are generally well known and considered as state-of-the-art (ref GB 2 040 738 A). In the case of direct application, the moving surface includes an outer surface of a material web such as paper or cardboard. In the case of indirect application, the moving surface includes an outer surface of a transfer element, preferably a transfer roll which transfers the coating medium onto the material web. In order to achieve a uniform coating with such a coating mechanism, a coater must be uniformly supplied with a coating medium. That means that the pressure of the incoming coating medium has to be equally applied onto the coater across the entire coating width so that the coater is uniformly lifted off the moving surface to form a metering slot of the desired width. This applies to the application of a coater blade, as well as to a smooth or profiled coater rod. The xe2x80x9cprofilexe2x80x9d of the coater rod can be generated by way of wire sheathing, machining, etching, or forming of impressions onto its surface.
GB 2 040 738 does not deal with the problem of achieving a uniform coating, but introduces a concept that is capable of compensating for the surface wear of the coater bed supporting the coater rod. It should be further noted that GB 2 040 738 describes the coating medium being supplied to the coater as already having taken the shape of a thin film. An older, re-published document DE 197 23 458 A1 discloses a coating mechanism, which includes an accumulator chamber positioned downstream of the coater, whose boundaries are formed by an accumulator chamber inlet limiting element at the moving surface entrance and a coater at the moving surface exit. The main purpose of the accumulator chamber inlet limiting element is to keep air bubbles from entering the accumulator chamber.
It cannot be discerned from GB 2 040 738 A that the coating quality can be influenced by the accumulator chamber positioned in front of the coater and by altering its geometry. The movement of the limiting surface of the coater facing the moving surface towards the moving surface serves to balance the wear in the intake area of the coater rod. This is only possible because the coating medium is supplied in form of a thin film and does not accumulate or xe2x80x9cback upxe2x80x9d in front of the coater rod.
In terms of the subject matter, DE 197 213 458 differs by the fact that the accumulator chamber is open on one side; that means it is not bound by an accumulator chamber limiting element.
The present invention provides a coating mechanism that is capable of uniformly applying a liquid or viscous coating medium onto a moving surface. A machine is designed for direct or indirect application of a liquid or viscous coating medium onto a moving surface. In the case of direct application, the moving surface is an outer surface of a material web, such as paper or cardboard. In the case of indirect application, the moving surface is an outer surface of a transfer element, preferably a transfer roll designed to transfer the coating medium onto a material web. The machine further includes a coating mechanism with a coater bed and a coater, which, together with the moving surface, establish a metering slot.
A limiting surface of the coater positioned at the inlet of the moving surface forms an accumulator chamber designed to accumulate or collect the coating medium transported by the moving surface. This chamber includes an opening at the side of the chamber opposing the feed direction of the moving surface. This chamber gradually decreases in volume in the feed direction of the moving surface. Such a chamber further includes an adjusting mechanism in order to alter the relative position of the limiting surface with respect to the moving surface and to thereby alter the shape of the accumulator chamber.
The present invention has distinct advantages over prior art coating mechanisms. By providing a chamber ahead of the coater in which the coating medium accumulates, a transverse flow patterns builds upstream of the coater, i.e., the flow has components in a direction perpendicular to the moving surface. This cross-flow leads to a more even distribution of the coating medium across the width of the moving surface on one hand, and, on the other hand, to a more balanced pressure distribution of the accumulated coating medium. This has the consequence that the coater receives the coating medium more evenly, resulting in overall improvements in coating quality.
An additional advantage of the accumulator chamber can be realized by changing the geometry of the accumulator chamber by use of an adjusting mechanism. This alters the hydrodynamic pressure in a very specific manner, which, in turn, affects the coating thickness without having to change the feed speed or the viscosity of the coating medium.
When applying a coating mechanism that employs a coater rod, the provision of the accumulator chamber has the further advantage of minimizing the influence of the coater rod diameter, i.e., the surface curvature of the coater rod, on the hydrodynamic pressure acting upon it. More specifically, the combination of a coater rod having a small diameter and a limiting surface designed as described by the present invention can result in conditions that are normally only achievable with very large diameter coater rods. This allows for the advantages of coater rods with small diameters, such as the easier handling, lower manufacturing cost, etc., to be combined with the advantages of large diameter coaters such as the increased amount of coating medium that can be applied onto the moving surface per unit time, as well as the lower pressure being exerted onto the moving surface, etc.
Additionally, when applying the coating mechanism in accordance to the present invention, it requires only a reduced number of coater rods with varying diameters to cover the full operating spectrum of the coating procedure.
Finally, the even distribution of the coating medium in the accumulator chamber, and therefore, the improved pressure distribution in the coating medium, allows the pre-metering amount to be lowered, which, in turn, lowers the total amount of circulating coating medium and, hence, the required pumping power.
It should be noted here that the above mentioned optimization of the operating conditions can be achieved not only with smooth coater rods, but also with profiled coater rods. An optimum color distribution can be achieved when using jets (for example slotted jets or spray jets, etc.) for the pre-metering of coating films in film presses.
In general, this coating mechanism can be applied in coating equipment, which is commercially available through the corporation of the applicant under the name xe2x80x9cSpeedsizerxe2x80x9d, xe2x80x9cSpeedCoaterxe2x80x9d and xe2x80x9cSpeedFlowxe2x80x9d. Further advantages include the capability of achieving targeted shear stresses of the coating medium in the accumulator chamber, as well as the capability of affecting the mold clamping force of the coater bed to avoid color circles on the coater rod or to avoid coater rod vibrations.
The above indicated advantages can be achieved especially when the length of the accumulator chamber, as measured in direction of feed, is between 2 mm and 100 mm, preferably between 5 mm and 50 mm, and/or when the width of the accumulator at the inlet is between 0.5 and approximately 5 mm, preferably between 0.5 mm and approximately 2 mm, as measured in a direction that is perpendicular to the direction of feed as well as perpendicular to the transverse direction of the moving surface. If the feed speed of the moving surface is relatively low, i.e. 900 m/min, an accumulator chamber length that is comparatively large with a relatively small inlet width can be applied. With an average feed speed of approximately 1000 m/min, the accumulator chamber length, as well as the inlet width, can also be mean values. In the case of higher feed speeds, especially when the speeds exceed 1500 m/min, a short accumulator chamber length having a large inlet width can be used. Of course, the above mentioned relative values are in reference to the absolute values of the accumulator length and inlet width stated at the beginning of this paragraph.
If the coating mechanism is further equipped with a distribution chamber adjacent to the inlet of the accumulator chamber, the cross-flows, which are required to balance the pressure in the incoming coating medium, can be kept away from the metering slot by instituting simple design considerations. This further improves the quality of the coating result. With this additional development of the present invention, the pressure balancing occurs initially in the distribution chamber, which is further removed from the metering slot. The coating medium is subsequently fed through the narrower accumulator nip to the metering gap.
The distribution chamber can have a length of between 5 mm and approximately 30 mm, for example, as measured in the direction of feed, and/or an inlet width ranging from approximately 4 mm to 11 mm, as measured in a direction that is perpendicular to the direction of feed as well as perpendicular to the transverse direction of the moving surface.
In order to simplify the altering of the relative position of the limiting surface, which bounds not only the accumulator chamber but also the distribution chamber, the adjusting mechanism can be designed to be capable of simultaneously altering the shape of the accumulator chamber, as well as that of the distribution chamber.
Altering the geometry of the accumulator chamber (and the distribution chamber) can be simply accomplished by adjusting a limiting surface of a coater bed. A coater bed has a base unit onto which the coater is attached, while the limiting surface is part of a tongue plate which is positioned at a distance relative to the base unit while being connected to it in a flexible manner. The adjusting mechanism can support itself on the base unit as well as on the coater bed.
Alternatively, the same effect can be achieved by rotating the coater bed by moving an adjusting mechanism about an axis positioned in the transverse direction relative to the moving surface. If, as an additional measure, the tongue plate is supported at its free end by a support element of the coating mechanism, the approaching and receding movements of the limiting surface of the coater bed at a point along the tongue plate near the coater are amplified as compared to a point along the tongue plate that is further removed from the coater, which, once again, has a favorable impact on the pressure distribution of the coating medium accumulating in the area ahead of the coater.
As an alternative to the above-described options detailing coater bed design and adjustment options, the coater bed can also be attached to a support element of the coating mechanism via a flexible web so that an approach or recession (with respect to the moving surface) of the coater bed surface defining in part the accumulator or distribution chamber can be achieved by moving the coater bed as a whole. The adjusting device can support itself on the coater bed as well as on the support element.
For the above-described design, which employs a coater rod to serve as a coater, the rod can have a diameter of between 10 and 38 mm, preferably approximately 24 mm, which is advantageous as far as handling is concerned.
In a further development of the present invention, at least one section of the adjoining limiting surface can be made flat. In order to achieve an optimum hydrodynamic interaction between the limiting surface and the coater rod, this flat section of the flat limiting surface can be positioned at a distance of up to 1 mm relative to an imaginary plane positioned tangentially to the coater rod and substantially parallel to the flat section of the limiting surface. Additionally, or alternatively, the flat section of the limiting surface can be positioned at an angle of up to 10 degrees relative to an imaginary plane positioned tangentially to the coater rod, allowing a smooth convergence in the accumulator/nip area, and thus avoiding the undesired generation of turbulences in the coating medium.
Additionally, or alternatively to the flat surface section, the limiting surface can also include a section which has the shape of a partial outer surface of a circular cylinder. Specifically, this circular cylinder can have a radius of between 10 mm and 600 mm, preferably approximately 50 mm.
In order to avoid deposits on the limiting surface, at least a part of the surface sections of the limiting surface can be connected by rounded-off transition sections.
As touched upon in a previous section of this text, with a coating mechanism employing a coater rod placed in a cavity of the coater bed in a such a manner that it is allowed to rotate, any changes to the relative position of the limiting surface and moving surface should not affect the support of the coater rod in its seat. This allows an independent adjustment of the coater rod mounting in the rod cavity on one hand and the geometry of the accumulator chamber on the other hand.
In order to facilitate a pressing of the coater rod against the moving surface and in order to be able to fix the position of the coater in the coater bed, an additional adjusting mechanism can be provided, which can be activated independently from the above-described adjusting mechanism. The terminology xe2x80x9cfixing the positionxe2x80x9d in this context describes a measure to secure the coater rod to keep it from falling out. Concurrently, though, it must be assured that the rod is still capable of rotating in its bed.
In order to respond to possible non-uniformities that remain in the coating, it is suggested that the minimum of one adjusting mechanism includes a plurality of adjusting elements distributed in the transverse direction of the machine, all of which are activated independently from each other. The adjusting elements can be activated in at least one of the following manners: electrically, hydraulically, pneumatically, hydro-pneumatically and manually. An especially simple design of the adjusting mechanism can be achieved when at least part of the adjusting elements have pneumatic hose units. Further, in view of achieving a satisfactory coating profile in the transverse direction, at least one adjusting element can contain a pneumatic hose that includes a plurality of individual pressure chambers.
The invention further relates to a process designed to apply a liquid or viscous coating medium onto a moving surface by use of a machine as it is described above. The process allows the coating pressure to be influenced or adjusted by altering the relative position of the limiting surface with respect to the moving surface, that is, by altering the shape or geometry of the accumulator chamber. With respect to the advantages and further development opportunities of this process, reference is made to the aforementioned discussion of the coating mechanism.
It should be especially highlighted here that the process, as described by this invention, lends itself to modify or adjust the transverse profile of the coating that is to be applied onto the moving surface by altering the relative position of the limiting surface with respect to the moving surface in specific zones of the application area.